1. Field of the Invention
The present invention relates to the biodegradation of a metal cyanide or a mixture of two or more metal cyanides and particularly an iron cyanide or mixture of iron cyanides.
2. Discussion of the Background
Coal gasification to produce methane gas was common practice across most of Europe, the UK and USA throughout the nineteenth century to the mid 1960s. This process has fallen into general disuse leaving an estimated 5,000 former gasworks contaminated with the by-products of gasification and its subsequent purification These contaminants typically include inorganics such as sulphur and cyanide containing compounds, metals, volatile aromatics, phenolics and poly aromatic hydrocarbons (PAHs). The removal of hydrogen cyanide and hydrogen sulphide, by passing the gas through oxide purifiers containing iron oxide, resulted in the production of spent oxide containing up to 45,000 mg Kg.sup.-1 of cyanide. Most of the cyanide is complexed with iron in the form of compounds such as Prussian Blue (Fe.sub.4 [Fe(CN).sub.6 ].sub.3), giving the spent oxide its characteristic blue colour. The solubility of Prussian Blue is strongly dependent on pH. Although highly stable and insoluble at low pH (typical of gasworks soil), above pH 4 the iron cyanide concentrations in groundwater, due to leaching and run-off, may exceed the Dutch maximum tolerated level of 200 .mu.g L.sup.-1 (equivalent to 1.28.times.10.sup.-6 mM) (Meeussen et al, 1992). With this consideration Meeussen et al (1992) have determined that all Prussian Blue should be removed before iron cyanide concentrations will fall to a tolerable level. Biological processes where toxic compounds undergo complete mineralisation are often cheaper, and deemed more environmentally aware than chemical processes to remove such toxic chemicals. Isolation of an organism that has the ability to degrade iron cyanide compounds including Prussian Blue is therefore highly desirable.
Stemphylium loti is a pathogenic fungus of the cyanogenic plant birdsfoot trefoil. The fungus has been shown to induce cyan EQU H.sub.2 O+HCN.fwdarw.HCONH.sub.2
Fry and Millar (1972) also showed that the enzyme has an optimum pH activity range of 7.0 to 9.0.
Cyanide hydratase has since been purified from a number of other fungi including Fusarium laterium (Cluness et al, 1993) and Gloeocercospora sorghi (Wang and Van Etten, 1992).
The bacterium Pseudomonas Fluorescens NCIMB 11764, first isolated by Harris and Knowles (1983a), utilises KCN under nitrogen limiting conditions at neutral conditions in fed batch. The key enzyme in this case is cyanide oxygenase (Harris and Knowles, 1983b), where the cyanide is converted to ammonia which is subsequently utilised in industrial processes. EQU HCN+O.sub.2.fwdarw.CO.sub.2 +NH3
Kunz et al (1992) later showed that cyanide hydratase is also present in a Pseudomonas species, although its activity is limited to higher concentrations of 20-50 mM cyanide.
A number of bacteria including Pseudomonas Fluorescens NCIMB 11764 (Rollinson et al, 1987), Pseudomonas putida BCN3 (Silva-Avalos et al, 199x) and Pseudomonas paucimobilis mudlock ATCC 39204 (see U.S. Pat. No: 4,461,834 to Mudder and Whitlock), have the ability to utilise moderately strong metal complexed cyanides in the form of nickel cyanide [Ni(CN).sub.4.sup.21 ] in nitrogen limited batch or continuous culture at neutral pH values. Again cyanides oxygenase activity was observed.
Despite the fact that a number of metallo cyanides, including Ni(CN).sub.4.sup.2- and Cu(CN).sub.4.sup.2-, have been shown to be biodegradable at neutral pHs (around pH7) by bacteria, no preculture of a micro-organism or a mixed culture of micro-organisms has been isolated with the ability to grow on iron cyanides. There is also no documentation of an organism capable of degrading metallo cyanides at a pH below about pH7. Likewise, although a number of fungi have been shown to tolerate cyanide, in the form of HCN, by detoxification using cyanide hydratase, a fungus has never been shown to grow on any metallo cyanide complex at pH7 or otherwise.